Magnetoresistive random-access memory (“MRAM”) is a non-volatile memory technology that stores data through magnetic storage elements. In a type of MRAM, the magnetic storage elements comprise two ferromagnetic plates or electrodes that can hold a magnetic field and are separated by a non-magnetic material, such as a non-magnetic metal or insulator. Such a structure is called a magnetic tunnel junction. In general, one of the plates has its magnetization pinned (i.e., a “reference layer”), meaning that this layer has a higher coercivity than the other layer and requires a larger magnetic field or spin-polarized current to change the orientation of its magnetization. The second plate is typically referred to as the free layer and its magnetization direction can be changed by a smaller magnetic field or spin-polarized current relative to the reference layer.
MRAM devices store information by changing the orientation of the magnetization of the free layer. In particular, based on whether the free layer is in a parallel or anti-parallel alignment relative to the reference layer, either a “1” or a “0” can be stored in each MRAM cell. Due to the spin-polarized electron tunneling effect, the electrical resistance of the cell change due to the orientation of the magnetic fields of the two layers. The cell's resistance will be different for the parallel and anti-parallel states and thus the cell's resistance can be used to distinguish between a “1” and a “0”. One important feature of MRAM devices is that they are non-volatile memory devices, since they maintain the information even when the power is off. The two plates can be sub-micron in lateral size and the magnetization direction can still be stable with respect to thermal fluctuations.
Spin transfer torque or spin transfer switching, uses spin-aligned (“polarized”) electrons to change the magnetization orientation of the free layer in the magnetic tunnel junction. In general, electrons possess a spin, a quantized number of angular momentum intrinsic to the electron. An electrical current is generally unpolarized, i.e., it consists of 50% spin up and 50% spin down electrons. Passing a current though a magnetic layer polarizes electrons with the spin orientation corresponding to the magnetization direction of the magnetic layer (i.e., polarizer), thus produces a spin-polarized current. If a spin-polarized current is passed to the magnetic region of a free layer in the magnetic tunnel junction device, the electrons will transfer a portion of their spin-angular momentum to the magnetization layer to produce a torque on the magnetization of the free layer. Thus, this spin transfer torque can switch the magnetization of the free layer, which, in effect, writes either a “1” or a “0” based on whether the free layer is in the parallel or anti-parallel states relative to the reference layer.
MRAM devices are considered as the next generation structures for wide range of memory applications. One MRAM technology uses a perpendicular magnetic tunnel junction device. In perpendicular MTJ devices, the free and reference layers are separated by a thin insulator layer for spin polarized tunneling. The free and reference layers have a magnetic direction that is perpendicular to their planes, thus creating a perpendicular magnetic tunnel junction (pMTJ). The pMTJ configuration may provide a lower critical switching current when compared to in-plane MTJ technology, simplified layer stack structure without need of using thick antiferromagnetic layers, and reduction of the device size below 40 nm.
FIG. 1 illustrates a pMTJ stack 100 for a conventional MRAM device. As shown, stack 100 includes one or more seed layers 110 provided at the bottom of stack 100 to initiate a desired crystalline growth in the above-deposited layers. A perpendicular synthetic antiferromagnetic layer (“pSAF layer”) 120 is disposed on top of the seed layers 110. MTJ 130 is deposited on top of synthetic antiferromagnetic (SAF) layer 120. MTJ 130 includes reference layer 132, which is a magnetic layer, a non-magnetic tunneling barrier layer (i.e., the insulator) 134, and the free layer 136, which is also a magnetic layer. It should be understood that reference layer 132 is actually part of SAF layer 120, but forms one of the ferromagnetic plates of MTJ 130 when the non-magnetic tunneling barrier layer 134 and free layer 136 are formed on reference layer 132. As shown in FIG. 1, magnetic reference layer 132 has a magnetization direction perpendicular to its plane. As also seen in FIG. 1, free layer 136 also has a magnetization direction perpendicular to its plane, but its direction can vary by 180 degrees.
The first magnetic layer 114 in the perpendicular SAF layer 120 is disposed over seed layer 110. Perpendicular SAF layer 120 also has an antiferromagnetic coupling layer 116 disposed over the first magnetic layer 114. As seen by the arrows in magnetic layers 114 and 132 of perpendicular SAF 120, layers 114 and 132 have a magnetic direction that is perpendicular to their respective planes. Furthermore, a nonmagnetic spacer 140 is disposed on top of MTJ 130 and a polarizer 150 may optionally be disposed on top of the nonmagnetic spacer 140. Polarizer 150 is a magnetic layer that has a magnetic direction in its plane, but is perpendicular to the magnetic direction of the reference layer 132 and free layer 136. Polarizer 150 is provided to polarize a current of electrons (“spin-aligned electrons”) applied to pMTJ structure 100. Further, one or more capping layers 160 can be provided on top of polarizer 150 to protect the layers below on MTJ stack 100. Finally, a hard mask 170 is deposited over capping layers 160 and is provided to pattern the underlying layers of the MTJ structure 100, using a reactive ion etch (RIE) process.
One of the key challenges in making pMTJ devices is the ability to create magnetically stable free and reference layers in the out-of-plane configuration for proper device operation. Such reference layers require high perpendicular magnetocrystalline anisotropy (PMA). In order to magnetically fix the reference layer, a perpendicular synthetic antiferromagnetic structure with high pinning fields (i.e., greater than 3 kG) must be formed.
In addition, achieving high perpendicular magnetic anisotropy (PMA) is a major challenge in development of perpendicular MRAM devices. Such devices require a fabrication of the pinned layers with PMA to serve as a reference layer in perpendicularly magnetized MTJs. Moreover, such structures need to withstand annealing temperatures up to 400 degrees Celsius for integration with underlying CMOS circuit structures used in conjunction with the MTJs.
Prior approaches to fabricate a pSAF such as SAF 120 used Co/Pd or Co/Pt multilayers. Perpendicular SAF structures made of Co/Pd or Co/Pt multilayers, however, fail when annealed at temperatures greater than 350 degrees Celsius by losing their perpendicular magnetic orientation and their antiparallel alignment in the pSAF structure. This is one of the significant limitations for device CMOS integration, which requires annealing at temperatures of 350 C or higher.
Thus, there is a need for pSAF for use with an MTJ device, where the pSAF has high PMA and can withstand a high annealing temperature so that the MTJ can be integrated with a CMOS device.